Escaping a C++ string - c++

Question

What's the easiest way to convert a C++ std::string to another std::string, which has all the unprintable characters escaped?
For example, for the string of two characters [0x61,0x01], the result string might be "a\x01" or "a%01".

Answer 1

Take a look at the Boost's String Algorithm Library. You can use its is_print classifier (together with its operator! overload) to pick out nonprintable characters, and its find_format() functions can replace those with whatever formatting you wish.
#include <iostream>
#include <boost/format.hpp>
#include <boost/algorithm/string.hpp>
struct character_escaper
{
template<typename FindResultT>
std::string operator()(const FindResultT& Match) const
{
std::string s;
for (typename FindResultT::const_iterator i = Match.begin();
i != Match.end();
i++) {
s += str(boost::format("\\x%02x") % static_cast<int>(*i));
}
return s;
}
};
int main (int argc, char **argv)
{
std::string s("a\x01");
boost::find_format_all(s, boost::token_finder(!boost::is_print()), character_escaper());
std::cout << s << std::endl;
return 0;
}

Answer 2

Assumes the execution character set is a superset of ASCII and CHAR_BIT is 8. For the OutIter pass a back_inserter (e.g. to a vector<char> or another string), ostream_iterator, or any other suitable output iterator.
template<class OutIter>
OutIter write_escaped(std::string const& s, OutIter out) {
*out++ = '"';
for (std::string::const_iterator i = s.begin(), end = s.end(); i != end; ++i) {
unsigned char c = *i;
if (' ' <= c and c <= '~' and c != '\\' and c != '"') {
*out++ = c;
}
else {
*out++ = '\\';
switch(c) {
case '"': *out++ = '"'; break;
case '\\': *out++ = '\\'; break;
case '\t': *out++ = 't'; break;
case '\r': *out++ = 'r'; break;
case '\n': *out++ = 'n'; break;
default:
char const* const hexdig = "0123456789ABCDEF";
*out++ = 'x';
*out++ = hexdig[c >> 4];
*out++ = hexdig[c & 0xF];
}
}
}
*out++ = '"';
return out;
}

Answer 3

Assuming that "easiest way" means short and yet easily understandable while not depending on any other resources (like libs) I would go this way:
#include <cctype>
#include <sstream>
// s is our escaped output string
std::string s = "";
// loop through all characters
for(char c : your_string)
{
// check if a given character is printable
// the cast is necessary to avoid undefined behaviour
if(isprint((unsigned char)c))
s += c;
else
{
std::stringstream stream;
// if the character is not printable
// we'll convert it to a hex string using a stringstream
// note that since char is signed we have to cast it to unsigned first
stream << std::hex << (unsigned int)(unsigned char)(c);
std::string code = stream.str();
s += std::string("\\x")+(code.size()<2?"0":"")+code;
// alternatively for URL encodings:
//s += std::string("%")+(code.size()<2?"0":"")+code;
}
}

Answer 4

One person's unprintable character is another's multi-byte character. So you'll have to define the encoding before you can work out what bytes map to what characters, and which of those is unprintable.

Answer 5

Have you seen the article about how to Generate Escaped String Output Using Spirit.Karma?